Method of heat treatment of housings

ABSTRACT

A method of heat treating housings wherein the low-stressed housing zones located in the side wall of the housing near spring pads are heated to 800* - 1250* C in the course of the time during which the housing zones are subjeted to maximum tensile stresses, and that which is located near spring pads in the lower wall of the housing are heated due to heat conduction to a temperature of not over 300* C after which said housing is cooled in the air in the course of at least 180 s.

United States Patent 91 Krendel et al.

[ Get. 29, 1974 METHOD OF HEAT TREATMENT OF HOUSINGS [76] Inventors:Leonid Avrelievich Krendel, ulitsa Gagarina 24, kv. 14, SerdobskPenzenskoi oblasti; Vladimir Filippovich Nikonov, ulitsa Avtozavodskaya1 1/1, kv. 125, Moscow; Viktor Alexeevich Ognevsky, ulitsa KrasnogoMayaka, 8, korpus 1, kv. 22, Moscow; Anatoly Georgievich Orlovsky,ulitsa Simferopolskaya, 14/2, kv. 21, Moscow; Grigory ArkadievichOstrovsky, Dimitrovskoe shosse, 41, kv. 125, Moscow; Valery EvseevichPalkin, Samarkandsky bulvar 6, korpus 2, kv. 190, Moscow; AnatolyVladimirovich Pakhomov, ulitsa Verkhnyaya Maslovka 28, kv. 10, Moscow;Anatoly Loginovich Stepin, ulitsa Birjulevskuya, 1, korpus 2, kv. 347,Moscow; Oleg Fedorovich Troilmov, ulitsa Chusovskaya 1 l, korpus 8, kv.92, Moscow; Konstantin Zakharovich Shepelyakovsky, ulitsaAvtozavodskaya, 6, kv. 125, Moscow; Alexandr Moiseevich Ryskind, ulitsaKirovogradskaya, 24, korpus l, kv. 161, Moscow; Isaak NakhimovichShklyarov, ulitsa Velozavodskaya, 9, kv. 24, Moscow; Moisei OsipovichRabin, Simonovsky val, korpus 1, kv. 119, Moscow; Veniamin Davydovic'hKalner, Lomonosovsky prospekt,

15, kv. 137, Moscow; Mikhail Mironovich Fishkis, Zvezdny bulvar, 5, kv.28, Moscow, all of USSR.

[22] Filed: Dec. 27, 1972 [21] Appl. No.: 319,044

Primary ExaminerC. Lovell Attorney. Agent, or Firm-Holmun & Stern [57]ABSTRACT A method of heat treating housings wherein the lowstressedhousing zones located in the side wall of the housing near spring padsare heated to 800 1250C in the course of the time during which thehousing zones are subjeted to maximum tensile stresses, and that whichis located near spring pads in the lower wall of the housing are heateddue to heat conduction to a temperature of not over 300C after whichsaid housing is cooled in the air in the course of at least 180 s.

3 Claims, 7 Drawing Figures METHOD OF HEAT TREATMENT OF HOUSINGS Thepresent invention relates to methods of the heat treatment of automobiledriving axles and can be utilized to advantage in automotive industry.

It is known that automobile driving axle housings are subjected to theeffect of asymmetric cyclic loads.

Under these circumstances the lower wall of the housing has zoneslocated close to the spring pads which are subjected to the effect ofmaximum tensile stresses. At the same time the working stresses appliedto the side walls near the neutral plane of the housing are very low.The zones of maximum stresses are strengthened by a known method of heattreatment in which the entire housing is hardened and then tempered.

In this known method the entire housing is heated to above the criticaltemperatures and then cooled in oil or water.

However, this through hardening of the housing leaves residual stressesin its superficial layers, i.e. at the points to be subjected to maximumtensile stresses. This impairs the fatigue strength of the housing.Another disadvantage of the known method for heat treatment lies in thehigh cost of the process due to a large consumption of electric powerfor heating the housing height.

and the high expenditures for refractory materials and quenching oil.

Besides, the known method of heat treatment calls for extensiveproduction areas.

An object of the present invention resides in the elimination of theaforesaid disadvantages.

The main object of the present invention is to provide a method of heattreating automobile driving axle housings which would produce residualcompressive stresses at the points to be subjected to maximum cyclictensile stresses, thereby extending the overall service life of thehousings.

This object is accomplished by providing a heat treatment method forautomobile driving axle housings which are to be subjected to the effectof asymmetric cyclic loads and which have zones located close to thespring pads on the lower wall of the housings carrying the maximumtensile stresses, and other zones located in the middle of the sidewalls and carrying low stresses wherein, according to the invention, thelow-stressed zones are heated to from 800 to 1250C for a certain timeperiod during which the zones being subjected to maximum tensilestresses are heated by conduction to not over 300C after which theentire housing is cooled in the air for at least 180 s.

This method of heat treatment improves the strength and durability ofthe housing and cuts down the consumption of refractories, quenching oiland electric power.

According to the invention, the low-stressed housing zones are heatedfor 180 s or less.

This time is sufficient to prevent the zones being subjected to maximumstresses from being heated in excess of 300C due to heat conduction.

Before heating the low-stressed zones of the housing it is possible,according to the invention, to harden the zones to be subjected tomaximum tensile stresses.

The hardening of these zones will strengthen the lower wall stillfurther.

These dimensions of the afore-mentioned zones of the housing will ensurea maximum increase in the housing life.

The embodiments of the invention are set forth with reference to theaccompanying drawings in which:

FIG. 1 is a diagrammatic view of the housing subjected to heat treatmentaccording to the invention;

FIG. 2 is a diagram of bending moments acting on the housing in service;

FIG. 3 is a section taken along line IIIlII in FIG. 1;

FIG. 4 is a section taken along line IV-IV in FIG. 1;

FIG. 5 is a diagram showing the distribution in height of stressesapplied to the housing in service;

FIG. 6 is a housing heating chart used in the heat treatment methodaccording to the invention;

FIG. 7 is a diagram showing the distribution of residual stresses on thelower wall of the housing after heat treatment according to theinvention.

The housing 1 (FIG. 1) of the automobile driving axle rests on wheels atpoints A and A and points B and B nearby carries the weight of theautomobile through spring pads.

A diagram of the bending moments acting in various sections of housing Iis shown in curve m (FIG. 2). The bending moment (curve m) grows frompoints A and A to points B and B respectively, whereas the pointsbetween B and B it has a constant maximum value. However, the section ofthe housing 1 between these points varies greatly. The section andresistance of the housing grow sharply in its middle part where thefinal drive is installed. As a result, the stresses acting in the middleof the housing are rather low and the danger of failure is negligible.

The lower wall 2 (FIGS. 3, 4) of the housing I is most susceptible tofatigue failure in the zones located close to the spring pads,particularly so near the transverse welds 3 where there is a highconcentration of stresses.

The same sections in the side walls 4 (FIGS. 3, 4) of the housing 1 nearthe longitudinal axis 5 (FIG. 1) have zones 6 subjected also being tolow stresses in service.

Indeed, curve n (FIG. 5) indicating the distribution of stresses alongthe height of the side wall 4 (FIGS. 3, 4) of the housing shows that thestresses pass through the zero value near the longitudinal axis of theside wall.

According to the invention, the low-stressed zones 6 (FIG. 1) are heatedto from 800 to l250C. The best solution is when the zones 6 on both sidewalls 4 are located above zones FG which are subjected to the effect ofmaximum tensile stresses, and at some distance from the housings lowerwall, said distance being equal to 0.4-0.7 of the housing height.

The height of the zones 6 is 0.2-0.5 of that of the housing heightwhereas their length depends on the length of the housing zonessusceptible to failure and may vary, correspondingly, to from I to 4 ofthe housing heights near the spring pads. Such localized heating can beeffected with the aid of any conventional means, such as inductioncurrents, contact electrical heating or gas torch means.

Assuming that the housing 1 is I20 mm high and the zone of probablefailure located on its lower wall 2 is 250 mm long, thus the zones 6would be located at a height of 80 mm and will be 50 mm wide and 300 mmlong.

According to the invention, the zones 6 (FIG. I) are heated to from 800to l250C within the time period for which the zones FG being subjectedto maximum tensile stresses cannot be heated by conduction above 300C.

FIG. 6 shows the heating curves of the zones 6 and FG (FIG. 1) of thehousing 1, with the horizontal axis indicating the time in seconds whilethe vertical one gives temperature in degrees Centigrade.

As we can see from FIG. 6, the low-stressed zones 6 (FIG. 1) reaches apeak when heated for 65 seconds to I IC (curve f in FIG. 6) while thezones FG (FIG. 1) when subjected to maximum tensile stresses are heatedby conduction only to 150C (curve g in FIG. 6).

The metal of the zones 6 being heated to l I0OC expands but the adjacentless-heated zones (the temperature difference exceeding 900C) opposethis expansion thereby causing plastic deformation of the heated metal.

After slow cooling, the metal in the zones 6 which were heated to 1 100Cwill be relatively smaller in size than that in the zones which remainedcomparatively cool.

As a result, the metal of the heated zones will be stretched while thatin the adjacent zones being subjected to maximum tensile stresses willbe compressed.

It thus follows from the above that a large difference in the heatingtemperatures of the zones being subjected to low and maximum tensilestresses is an indispensable prerequisite for the formation of stresses.

When the heating of the zones 6 is discontinued, the entire housing iscooled in the air over a period of at least 180 s.

During this period the temperature of the zones 6 (FIG. 1) drops (curvef in FIG. 6) while that of the zones FG (FIG. I) continues to rise(curve gin FIG. 6) due to the conduction of heat from the zones 6.

Thus, slow cooling in the air, as shown in FIG. 6, equalizes thetemperature of all the housing zones at a temperature of about 400C.

The low heating temperature of the entire housing (400C) makes itpossible to retain a comparatively high level of residual stresses.

If, however. the zones are heated to llO0C and quickly cooled in wateror oil. they may be hardened which will increase their volume andstretch the zones being subjected to maximum tensile stresses this beingunallowable.

The distribution of the residual stresses along the housing sectionafter the heating described above and after cooling in the air, curve Kas measured by electrical resistance strain gauges, is shown in FIG. 7.The residual stresses marked by the numerals in FIG. 7 are expressed inkg/mm with the compressive stresses being shown by the plus sign (-I-).It appears from FIG. 7 that the heat treatment according to theinvention produces the desired distribution of residual stresses on thelower wall 2 which is to be subjected to maximum tensile stresses andwill cause residual compressive stresses reaching 15 kg/mm".

The residual tensile stresses arising in the heated zones 6 (FIG. 1) arenot dangerous since said zone 6 is located in the region of low tensileor compressive stresses; at the same time the residual compressivestresses arising on the lower wall 2 of the housing weaken the tensilestresses applied to the housing during operation, thus prolonging thehousing life.

The strength characteristics of the housing can be further improved ifthe zones FG (FIG. 1) to be subjected to maximum tensile stresses arehardened before heating of the zones 6.

In this case the lower wall 2 (FIGS. 1, 3, 4) of the housing in the zonePG and the adjoining zones of its side walls 4 (FIGS. 1, 3, 4) becomehardened throughout, with of said zones being from 0.2 to 0.3 of that ofthe housing height. The zones of the lower wall and the side wall zonesadjacent thereto may be hardened by known methods of heating above theside critical points, usually above 850C for steel materials employed inthe production of housings, followed by immediate cooling in oil orwater.

The localized heating for hardening may be effected by any known means,such as by induction currents.

This kind of hardening produces residual compressive stresses in thelower wall 2 of the housing in the zones of maximal service stresses.The residual compressive stresses are formed in the lower wall duringlocalized hardening thereof, due to a great volume of martensitedeveloped as a result of hardening relative to the surrounding initialstructure. Subsequent heating of the low-stressed zones 6 (FIG. 1)imposes additional residual 'compressive stresses on the zones FG.

There is no need for special tempering of the hardened zones of thehousing lower wall 2.

The heating of the zones 6 to high temperatures followed by air coolingresults in the heating of the entire housing 1 to I00C which issufficient for tempering the hardened zones.

This method of heat treatment produces favourable distribution ofstrength characteristics and residual stresses throughout the housingsection; thus, the lower wall 2 (FIGS. 1, 3, 4) of the housing (in thezones of probable failure) has the structure of tempered martensitecharacterized by a high strength and high yield limits and possessessufficient residual compressive stresses which raise the resistance ofthe housing to asymmetric cyclic loads.

In some cases the low-stressed zones 6 (FIG. 1) can be heated afterhardening the entire housing.

In this case, just as in the local hardening, there is no need fortempering the housing in the zones of maxi mum tensile stresses for thesame reasons as above.

The service life of the housings after the heat treatment according tothe present invention is more than doubled as compared with the life ofthe housings subjected to heat treatment according to the methods knownheretofore.

Tests have shown that the housings heat-treated according to the knownprocedure which are subjected to cyclic loads varying from 0 to 17 tonshave endured from 700,000 to l,000,000 cycles whereas the housingsheat-treated according to the present invention have height from thehousing lower wall, said zones having a length of from 1 to 4 times thehousing height near said spring pads and a height of from 0.2 to 0.5 ofsaid housing height near the spring pads, said zones of the housing sidewalls being heated at said temperature for not more than 180 secondswherein the zones of said lower wall of the housing carrying maximumtensile stresses are heated by conduction to a temperature of not above300C, and cooling the housing in air'for at least l80 sec.

2. The method of claim 1 wherein said heating of said zones of thehousing is carried out after hardening of the zones of the lower wallsand side walls adjacent thereto located near the spring pads and havinga length of from i to 4 times the housing height near said spring padsand a height of from 0.2 to 0.3 of the housing height near said springpads for said zones of the side walls.

3. The method of claim 1 wherein the heating of said zones of thehousing is carried out after hardening of the entire housing.

1. A METHOD OF HEAT TREATMENT OF AUTOMOBILE DRIVING AXIE HOUSINGSSUBJECTED TO ASYMMETRIC CYCLIC LOADS WHICH COMPRISES HEATING THE HOUSINGSIDE WALL ZONES ADJACENT TO SPRING PADS TO A TEMPERATURE OF 800* TO1250*C AT A HEIGHT OF FROM 0.4 TO 0.7 OF THE HOUSING HEIGHT FROM THEHOUSING LOWER WALL, SAID ZONES HAVING A LENGTH OF FROM 1 TO 4 TIMES THEHOUSING HEIGHT NEAR SAID SPRING PADS AND A HEIGHT OF FROM 0.2 TO 0.5 OFSAID HOUSING HEIGHT NEAR THE SPRING PADS, SAND ZONES OF THE
 2. Themethod of claim 1 wherein said heating of said zones of the housing iscarried out after hardening of the zones of the lower walls and sidewalls adjacent thereto located near the spring pads and having a lengthof from 1 to 4 times the housing height near said spring pads and aheight of from 0.2 to 0.3 of the housing height near said spring padsfor said zones of the side walls.
 3. The method of claim 1 wherein theheating of said zones of the housing is carried out after hardening ofthe entire housing.